Allow vTPM state encryption when swtpm_setup and swtpm support
passing a passphrase using a file descriptor.

This patch enables the encryption of the vTPM state only. It does
not encrypt the state during migration, so the destination secret
does not need to have the same password at this point.
Signed-off-by: NStefan Berger <stefanb@linux.ibm.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>